#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N = 5e4 + 10;

int t, n, m;
int a[N];

int check(int x)
{
	int ans = 0;
	for(int i = 0;i <= n;i ++)
	{
		int j = i + 1;
		while(j <= n && a[j] - a[i] < x) j ++;
		ans += j - i - 1;
		i = j - 1;
	}
	return ans;
}

int main()
{
	cin >> t >> n >> m;
	for(int i = 1;i <= n;i ++) cin >> a[i];
	a[n + 1] = t;
	n ++;
	int l = 0, r = t;
	while(l < r)
	{
		int mid = l + r + 1 >> 1;
		if(check(mid) <= m) l = mid;
		else r = mid - 1;
	}
	cout << l << endl;
	return 0;
}